In the prior art, multiple disc switching elements of a drive train are actuated in such a way that the torque that is to be transmitted by the switching element is adjusted by means the engine torque of an electric motor via the clutch pressure of a pressure actuator, or via the attractive force of an electromagnet. The disadvantage of the known torque, pressure, or force-based actuation methods is the occurrence of strong hysteresis effects which have an adverse effect on the accuracy of the torque to be transmitted.
The applicant's German patent application with the file number 10 2006 056 515.0, which has not yet been published, discloses a method of actuating a clutch in which the torque to be transmitted by the clutch is continuously adjusted by electromechanical, hydraulic, or electromagnetic means, the torque that is to be transmitted being adjusted using a position-dependent clutch torque set-point value. In this way, hysteresis effects can largely be avoided.
With the known method, there is a proportional dependence for the electromechanical adjustment of the torque between the rotational angle of the electric engine and the clutch position, i.e. the position of a movable clutch part, so that the clutch position is controlled by means of the electric motor position control. Here, the rotary motion of the electric motor is transmitted via a spur-gear stage and a ball ramp to the movable clutch part.
Systems of this type with position-controlled actuation of a multiple-disc clutch or a multiple-disc brake are subject to changes caused by the axial wear on the discs, which also results in changes in the counterforce gradient or, as the case may be, the rigidity of the system. In addition, in case of electromechanical actuation using a spur-gear stage and a ball ramp, there are disturbing effects due to different temperature characteristics, hysteresis effects, or the notchiness of the system caused by the engagement of the gear teeth with the spur-toothed spur gear stage between electric motor and the ball ramp.